A typical inkjet printer uses one or more printheads to form an ink image on an image receiving surface. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving surface to form an image. The image receiving surface may be a continuous web of recording media, a series of media sheets, or the image receiving surface may be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that eject ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The magnitude, frequency, and/or duration of the firing signals affect the amount of ink ejected in each drop. The firing signal is generated by a printhead controller in accordance with image data. An inkjet printer forms an ink image with reference to electronic image data by printing a pattern of individual ink drops at particular locations on the image receiving surface. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to the electronic image data.
In order for the printed ink images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads must be registered with reference to the imaging surface and with the other printheads in the printer. Registration of printheads is a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the orientation of a printhead with reference to the imaging surface and with reference to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with a width across the image receiving surface and that all of the inkjet ejectors in the printhead are operational. The presumptions regarding the orientations of the printheads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the printheads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the printheads so they better conform to the presumed conditions for printing or to compensate for the deviations of the printheads from the presumed conditions.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving surface is moving as the imaging surface passes the printhead to receive the ejected ink and “cross-process direction” refers to the direction across the width of the image receiving surface. In order to analyze a printed image, a test pattern needs to be generated so determinations can be made as to whether the inkjets operated to eject ink did, in fact, eject ink and whether the ejected ink landed where the ink would have landed if the printhead was oriented correctly with reference to the image receiving surface and the other printheads in the printer. In some printing systems, an image of a printed test pattern is generated by using image data of a test pattern to eject ink onto media or by transferring such a printed test pattern from an image receiving surface onto media, discharging the media from the system, and then scanning the image with a flatbed scanner or other known offline imaging device. This method of generating image data of the printed test pattern suffers from the inability to analyze the test pattern in situ and from the inaccuracies imposed by the external scanner.
In some printers, the ink drops that form the test pattern are printed directly onto a print medium such as an elongated paper web. The test patterns are formed in “inter-document zones,” which are gaps between printed pages on the web. In some embodiments, the inter-document zones are later removed from the finished printed material with cutting devices. In a cut-sheet printer, however, each printed sheet includes an ink image and the test pattern cannot be easily removed from each individual sheet through a cutting process. Instead, the printer transfers ink drops forming test patterns onto the same sheets that receive printed images. The test patterns can negatively impact the quality of the images that are printed on the sheets or on the sheets of paper web printers where the inter-document zones are not removed after the printing process. Consequently, improvements to printers to enable detection of inoperable inkjets and register printheads while maintaining the quality of printed images would be beneficial.